1. Field of the Invention
The present invention relates generally to an electric power source which contains a plurality of batteries in a battery case and pertains specifically to a power source which is used to supply power to a motor mainly for driving a vehicle.
2. Description of the Related Art
An electric motor vehicle, such as an electric car and a hybrid car, which is designed to travel optionally with an internal combustion engine or with an electric motor, uses an electric power source, having a multitude of batteries interconnected, as a power unit for supplying electric power to a drive motor.
An electric power source used for this kind of application is designed to provide a high output voltage so that a large amount of electricity may be supplied to a motor which requires a high power. In order to satisfy such a design need, a multitude of batteries, interconnected in series, are contained in a holder case. For example, a currently commercially available power source mounted to a hybrid car has hundreds of batteries interconnected in series to generate a high output voltage. Such power source is designed to have five or six pieces of batteries interconnected in series to form a battery module, and further a multitude of such battery modules are interconnected in series within a holder case.
In the electric power source mounted to an electric motor vehicle such as a hybrid car, when the vehicle needs a burst of speed, a large current is discharged to accelerate the motor, and when the vehicle is slowed down or when the vehicle travels down a slope, a large current is charged by means of a regenerative brake. Such a discharging and charging cycle may often cause the battery to be heated to a considerably high temperature. In addition, when the battery is used at higher temperatures, like in a summer season, the battery temperature tends to be elevated to even higher degrees. In view of these factors, when a power source contains a multitude of batteries in its holder case, it is vital to cool each of the contained batteries both effectively and uniformly. This is because various disadvantages are likely to occur when a temperature difference exists among the batteries being cooled. For example, a battery having undergone a high temperature tends to degrade, and thus results in a reduced amount of real charge capacity for reaching a full charge. When a battery with a reduced amount of real charge capacity is interconnected in series to be charged and discharged with the same current, the battery is very likely to be overcharged or over-discharged. This happens when a full charge capacity and a full discharge capacity have become smaller. A remarkable decrease is caused to a battery in its property or performance through an overcharge and over-discharge, so that a battery with a reduced, real amount of charge capacity is accelerated into degradation. Especially when the battery temperature is elevated to higher degrees, the battery is even more likely to be degraded. For these reasons, when an electric power source contains a multitude of batteries in a holder case, it is important to uniformly cool all the batteries so that a temperature irregularity may be prevented.
There have been developed a variety of battery structures to overcome such drawbacks resulting from the temperature irregularity. Refer to Japanese Laid-Open Patent Publication Nos. 2001-313090, 2002-50412 and 1999-329518.